How to transform flat face to concave one - model palate?
Isolate the ground plane (or remove it and create a new one. Then subdivide it and use Proportional Editing (Keyboard O in edit mode) to shape it to your liking.
EDIT: here the illustration: Youtube - Blender teeth modeling. It's just one way though. There might be better ways depending on the existing topology and what kind of quality you are trying to achieve.